1. Field of the Invention
The present invention relates to magnetic sensors in which a sense current is applied to a magnetosensitive film to sense a change in external magnetic field through a change in resistance value (voltage value) of the film.
2. Description of the Related Art
A magnetic sensor is conventionally used as a recording/reproduction magnetic head of a hard disk drive mounted principally in a computer. This conventional recording/reproduction magnetic head senses an external magnetic field by means of an induced current generated in a coil. Recently, however, with increasing demands on high storage density and high processing speed, a magnetic sensor which senses an external magnetic field itself is often used. This sensor uses the magnetoresistance (MR) effect. Furthermore, a magnetic head using the giant magnetoresistance (GMR) effect has appeared.
As the recording density of a hard disk drive increases, the 1-bit recording area reduces, and the magnetic field generated decreases. The recording densities of currently commercially available hard disk drives are around 10 Gbits/in2. However, the recording density rise nearly doubles every year. This requires a magnetic head to control finer magnetic fields and sense smaller changes in external magnetic field.
At present, a spin valve type GMR film is used. This spin valve GMR film has a magnetic layer whose magnetization direction is fixed and a magnetic layer whose magnetization direction can freely change. The electrical resistance changes in accordance with the angle that the magnetization directions in these two magnetic layers make.
When this magnetic head is used, a sense current is made to flow in parallel with the film surface of the GMR film and sensed as a change in the resistance value (the voltage value: the output value) resulting from the external magnetic field. In this magnetic head having a CIP (Current In the Plane) structure in which a sense current is supplied in parallel with the film surface of the GMR film, the output value lowers when the element width (accurately, the effective core width) decreases. If the sense current amount is increased, a large output value is obtained by the Ohm's law. However, this sense current amount is limited because the generation of heat or the like is unavoidable.
Also, the CIP magnetic head requires insulating layers between the upper and lower magnetic shields. The distance between the magnetic shields is (thickness of GMR film+thickness of insulating layer×2). Since the lower limit of the insulating layer thickness is presently 20 nm, the distance between the magnetic shields is (thickness of GMR film+40 nm). If the lengths of recording bits on a recording medium are shortened, it becomes difficult to sense these bits. Therefore, the distance between the magnetic shields cannot be reduced to 40 nm or less at present.
From the foregoing, the CIP magnetic head using the spin valve GMR film can presumably achieve a recording density of up to 20 to 40 Gbits/in2. The upper limit is 60 Gbits/in2 even when the latest technique using specular scattering is applied.
The recording density of hard disk drives is abruptly increasing, so a recording density of 80 Gbits/in2 is probably required in 2002. For the above reasons, it is extremely difficult for the CIP magnetic head using the spin valve GMR film to achieve a high recording density of 80 Gbits/in2 or more.
To solve these problems, a magnetic head which has a CPP (Current Perpendicular to the Plane) structure in which a sense current is supplied in a direction containing at least a component perpendicular to the film surfaces of the MR film is regarded as promising. This magnetic head having the CCP structure shows a resistance change about twice that of the CIP structure at room temperature, so a large output can be expected. In this structure, the GMR film is not restricted to a multilayer GMR film. For example, a spin valve film or a coercive force difference type multilayer film can be used.
Another great advantage of the CPP magnetic head is that the output value increases as the sectional area (=core width of CIP structure×height) of a portion of the GMR film through which a sense current passes decreases. A high output value is obtained by decreasing those surfaces of upper and lower terminals sandwiching the MR film, which oppose the MR film surfaces, by using this property.
The use of a magnetic head having a tunnel MR (TMR) structure in which an insulating layer is sandwiched between two magnetic layers is similarly pursued. In this structure, a tunnel current passing through the insulating layer changes in accordance with the magnetization direction in each magnetic layer. Accordingly, the structure shows a large resistance change and has a high output value. In the magnetic head having this TMR structure, a current flows in the order of magnetic layer→insulating layer→magnetic layer. Additionally, the TMR structure has advantages analogous to those of the CPP structure. Hence, the TMR structure can be regarded as one type of CPP structure.
As described above, the CPP magnetic head is expected to replace the CIP magnetic head. However, this CPP magnetic head has not been put into practical use yet because it has the following several problems.
The following problems are particularly notable when the sizes of those surfaces of the upper and lower terminals, which oppose the GMR film surfaces, are decreased to further increase the output of the CPP structure magnetic head.
(1) Element fabrication processes are complicated and require high accuracy.
A series of processes of film formation, resist formation, photolithography, etching, and resist removal must be performed several times. In particular, when the sizes of the opposing surfaces of the upper and lower terminals are to be decreased, it is essential to form insulating layers corresponding to the two terminals. This formation is very cumbersome and time-consuming. Additionally, in this case those portions of the opposing surfaces of these upper and lower terminals, which overlap correspond to a CPP portion which contributes to the output. Hence, an extremely high alignment accuracy is necessary in resist formation, and this makes a desired output very difficult to obtain.
(2) The characteristics are difficult to evaluate unless the size of the CCP portion is around 1 μm or on the Order of Submicrons.
If the size of the CCP portion is 3 μm or more, a voltage with respect to a sense current is measured as a negative value owing to the influence of the current distribution, although this also depends on the element structure, material, and the like. Under this influence, the MR ratio takes a very large value around 3 μm. This prevents the application of the conventional evaluation standards.
(3) The characteristics are readily influenced by the quality of element fabrication processes.
Although this is also a problem of the magnetic head having the CIP structure, the problem is more notable in the CPP structure. When a GMR film and insulating layers are formed, the MR characteristics largely change in accordance with the sectional shape and the condition of burrs produced. This makes it difficult to specify the cause of a defective product.